In designing a contact block, it is important to keep the “pusher return force” in these contact blocks as low as possible, so that momentary selector switches can operate as many contact blocks as possible. To operate properly, a compression spring must rotate a cam that in turn forces the pusher into the contact block by overcoming the pusher return force. While most pushbuttons and selector switches can operate eight contact blocks simultaneously (two stacks of four deep), the present Siemens class 52 momentary selector switches are limited to operating four contact blocks.
Achieving a lower spring rate allows a lower compressed pusher return force because (i) the force applied by the return spring when the pusher has been forced into the contact block is the force applied by the return spring when the pusher is extended plus the pusher travel multiplied by the spring rate and (ii) the force applied by the return spring when the pusher is extended and the pusher travel are essentially fixed.
While a lower spring rate can be achieved by adding extra turns to a spring, the extra turns, however, increase the solid height of the spring. Consequently, when starting with a spring whose rate is already minimized, a lower spring rate can be further achieved by increasing the maximum allowable solid height of the spring. The top of the return spring of conventional contact blocks (which is mounted inside the pusher) requires clearance for the end of an extended pusher on a second contact block stack-mounted to the first contact block. This mounting configuration limits the solid height. Accordingly, there is a need for a contact block assembly and a method of assembling a contact block assembly with a return spring positioned outside the pusher.